Sewing machines of this type are widely distributed and their method of operation is well known. In practical terms, taking a single-needle machine as an example for the sake of simplicity, the stitches are formed by interlocking an upper thread fed by the needle which moves with reciprocating action through the piece to be sewn, with a lower thread unwinding from a bobbin inside a shuttle or crochet which moves with a reciprocating or rotary action through a loop formed by the upper thread. When the two threads have formed the stitch, they are tensioned by a tensioning device and another device advances the piece being sewn by a stitch-length.
The terms "shuttle" or "crochet" used with reference to sewing machines generally refer to a rotating or oscillating mechanism with a hook which receives and hooks the upper thread carried by the needle, forms a loop and passes it around a bobbin-case which is housed in the shuttle and houses in turn a little bobbin on which the lower thread is wound. These elements are arranged to allow the loop to pass around the lower thread and the stitch to be formed once the loop is tightened. These operations are repeated at each revolution of the sewing machine thus effecting the sewing.
The device for tensioning the threads to form the stitch is usually a take-up lever which moves alternately up and down, drawing out the upper thread to supply thread for the formation of the loop on its downward stroke, and pulling the same thread in again on its upward stroke to tighten the loop formed by the shuttle.
All the actions of the moving parts, in particular the movements of the needle, the shuttle and the take-up lever, are synchronised in order to allow the correct operation of the machine.
One of the main limitations of the known sewing machines is that the lower bobbin thread is rapidly used up and the replacement operation is long, complex and troublesome; suffice it to recall that the bobbin must be changed on a standard industrial machine from 20 to 30 times during the working day.
It is known that the size of the shuttle, the bobbin-case and hence the bobbin are necessarily limited by the necessity of recovering the upper thread to close the loop. The limit is set by the stroke of the thread take-up lever which corresponds substantially to the length of the loop formed by the shuttle.
In the present art, various attempts have been made to overcome these limitations, providing in particular a number of passes of the upper thread between the same thread take-up lever and a series of fixed thread-guides in such a way as to multiply the length of upper thread recovered at each cycle. Some examples of the machines based on these ideas are described in the UK Patent Application N.sup.o GB 2027762, the French Patent N.sup.o FR 1000704 and the U.S. Pat. No. 4070976.
However, the simple multiplication of the thread recovered by multiple passes between a mobile thread take-up lever and fixed thread-guides has not always lead to correct closing of the loop. For this reason, the resultant stitching is substantially irregular, and of unsatisfactory quality. This is due to the fact that the solutions proposed in the prior art do not take account of the factors which influence the closing of the stitch.
Account must be taken of the extension of the upper thread as a result of the elasticity of the same when the thread take-up lever recovers the upper thread and closes the loop formed below the shuttle. The extension of the upper thread must be considered as a function of the total length of the portion of thread recovered during the cycle. This phenomenon increases in importance as the speed of sewing increases, hindering the efficient use in the industry of machines which use the multiplication of the recovered upper thread as described above.
Furthermore, the friction which is experienced by the upper thread as it passes repeatedly over the lever and the fixed guides causes differences in the tension of the thread upstream of any such pass and the tension downstream of the same pass. While faking all possible steps to reduce the friction of the thread (such as lapping the eye-holes or lining them with low-friction material), the total friction increases with the number of passes over the lever or guides, hindering correct control of the tension during the closing of the loop.
Another drawback of the sewing machine according to the prior art occurs when a particularly delicate thread, e.g. silk or the like, or particularly thin thread is used. In such cases, the thread take-up lever which recovers the upper thread exercises a tension on a particularly long section of the same thread. As a result, a certain section of upper thread is subjected to tension for a high number of cycles, often causing the thread to break. This drawback often means that continuous sewing for more than 30-40 cm without the upper thread breaking is not possible.